Although International Publication WO 00/42019 and International Publication WO 02/06270 describe a 6-arylphenanthridine compound as a PDE4 inhibitor, in which a cyclohexane ring is formed between positions 3 and 4 of a dihydroisoquinoline ring, while Japanese Patent Publication No. 2003-171381 describes a 6-arylfuroisoquinoline compound as an entry inhibitor, in which a dihydrofuran ring is formed between positions 7 and 8 of a dihydroisoquinoline ring, there are no descriptions of a 3-dihydroisoquinolin-1-yl quinoline compound in which the isoquinoline ring is not condensed with another ring, and there are no descriptions relating to an agrohorticultural antimicrobial agent. In addition, although the Indian Journal of Chemistry 1969, 7(10), 1010-1016, ibid 1970, 8(6), 505-508, ibid 1985, 24B(7), 737-746, and ibid 1986, 25B(10), 1072-1078 describe the synthesis of a 3-(dihydro(tetrahydro)isoquinolin-1-yl)quinoline compound, there is no description of a 3-(dihydro(tetrahydro)isoquinolin-1-yl)quinoline compound in which position 3 of the isoquinoline ring is substituted by two substituents, and there are no descriptions relating to an agrohorticultural antimicrobial agent. In this manner, the use of a 3-(dihydro(tetrahydro)isoquinolin-1-yl)quinoline compound, in which position 3 of the isoquinoline ring is substituted by two substituents, as an agrohorticultural antimicrobial agent is not known in the prior art.
As a result of conducting extensive studies on a 3-(dihydro(tetrahydro)isoquinolin-1-yl)quinoline compound, the inventors of the present invention found that a 3-(dihydro(tetrahydro)isoquinolin-1-yl)quinoline compound, in which position 3 of the isoquinoline ring is substituted by two substituents and other rings are not condensed with the isoquinoline ring, has superior antimicrobial activity against various plant diseases and is useful as an active ingredient of an agricultural chemical, and in particular, found that this compound is able to control rice blast (Pyricularia oryzae), which is a plant mold that frequently causes serious damage to agrohorticultural crops, as well as gray mold (Botrytis cinerea) in tomatoes, cucumbers and green beans, at low doses, thereby leading to completion of the present invention.